1. Field of the Invention
The invention relates to filter materials having improved filtration properties, which find application especially in vacuum cleaner bags.
2. Description of Related Art
The use of filter bags in vacuum cleaners is generally known. For such applications, the bags must satisfy a plurality of requirements, which in part are oppositely directed. Besides a high degree of dust separation, for coarse as well as fine dust, there are requirements for low air resistance, a low tendency to clog, as well as for mechanical stability.
A general problem with vacuum cleaner bags is their tendency to clog, when compact materials having mechanical filtration properties are used, such as paper or fine fiber spunbonded materials without voluminous build-up. To be sure, this compact build-up contributes to improving the dust barrier properties of a filter bag, but it makes the bag susceptible to fine dust, which penetrates these compact filter media and clogs them. Parameters such as suction efficiency stability and separation performance of a filter bag, such as a vacuum cleaner bag, tend to be compromises. This means that, in the case of improvement of suction efficiency stability, the separation performance of a vacuum cleaner bag can usually not be decisively improved.
Filter bags based on paper/melt-blown are described, for example, in DE-C-197 31 860 or EP-A-0 338 479; filter bags based on spunbond/melt-blown are known from EP-A-0 161 790.
From EP-A-0 960 645 inliner layers in a coarse filter embodiment are known, which protect the following, actual filtration layer from clogging by coarse dust. The vacuum bags described in this document are made of several layers which have one coarse filter layer made of various materials placed ahead of them, in order to raise the service life of the bag. In this connection, layers made of special papers and special non-wovens are combined with one another. The use of electrostatically chargeable or loaded fibers, for example, made of propylene fibers, in selected paper layers or layers of non-wovens is described.
From EP-A-0 246811 filter materials are known which are made of mixtures of polypropylene fibers and modacrylic (modified acrylic) fibers. Nonwoven materials made of these fiber mixtures can be used as vacuum cleaner bags. These fiber mixtures may be used especially well for filtration of fine dusts, on account of their strong electrostatic properties.
EP-A-0 674 933 describes the production of an air filter material having electrostatic effectiveness, which includes the production of a needle-punched card web made of selected fibers.
Starting from this related art, the present invention makes available an air filter material whose total separation performance (separation performance for coarse and fine dusts), as compared to the usual air filter materials, was significantly improved, without thereby substantially worsening the clogging tendency, as a matter of fact even improving it. The air filters according to the present invention are thus distinguished by improved overall separation performance, in comparison to known materials.
The present invention relates to a multi-layer air filter including a filter layer, preferably a filter layer containing a synthetic fiber, and a prefilter layer covering it on the dust-laden side, the prefilter layer having a laid dry and electrostatically effective nonwoven material, preferably a staple fiber nonwoven fabric, whose mass per unit area is 10 to 100 g/m2.